Method for fabricating gate oxide

ABSTRACT

A method for fabricating gate oxide includes a dilute wet oxidation process with additional nitrogen and moisture and an annealing process with a nitrogen base gas, wherein the volume of additional nitrogen is about 6-12  6-20 times of the volume of the additional moisture. The method according to the invention improves the electrical quality of the gate oxide by raising the Q bd  and by reducing the leakage current of the gate oxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application Ser.No. 87112101, filed Jul. 24, 1998, the full disclosure of which isincorporated herein by reference.

BACKGROUND Of THE INVENTION

1. Field of the Invention:

This invention relates to a method for fabricating a semiconductordevice, and more particularly, to a method for fabricating gate oxide,which improves the electrical quality of a gate oxide layer.

2. Description of Related Art:

Since gate oxide is one of the important components in ametal-oxide-semiconductor (MOS) device, the electrical quality of thegate oxide directly affects the quality of a MOS device. Conventionally,a wet oxidation process is usually performed on a gate oxide layer thatexceeds 200 Å in thickness. In a currentvery-large-semiconductor-integration (VLSI) process, the gate oxidelayer used in a MOS device, which has a thickness of about 100-200 Å, isnormally formed by a dry oxidation process in the presence of C₂H₂Cl₂vapor.

If the required thickness of gate oxide is thinner than 100 Å, perhapseven as thin as about 50 Å, such as the tunneling oxide within a flashmemory, the method for fabricating gate oxide is normally a dryoxidation. However, if the required thickness of the gate oxide isthinner than 50 Å, it is very difficult to obtain a high-quality gateoxide layer through a conventional oxidation process.

As shown in FIG. 1, which is a flowchart represents a conventionalmethod for fabricating gate oxide, an oxidation process is performed ona provided silicon substrate in step 10. A gate oxide layer is formed onthe silicon substrate by performing a thermal oxidation in anenvironment filled with oxygen at a temperature of about 900°-100° C.Then, in step 20, an annealing process is performed in an environmentfilled with gas, such as nitrogen or inert gases, at a temperature ofabout 900°-1100° C. This eliminates the stress within the gate oxidelayer caused by defects.

A gate oxide layer with a thickness of about 40 Å formed by theforegoing method has a measured breakdown charge (Q_(bd)) equal to orless than 5 coulombs per square centimeter. A measured leakage currentthrough the gate oxide layer is about 34.6×10⁻⁸ amperes under a 3.2-voltgate voltage. This shows that the electrical quality of the thin gateoxide is not acceptable

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide amethod for fabricating gate oxide that improves the electrical qualityof a gate oxide by raising the Q_(bd) and reducing the leakage currentof the gate oxide layer.

In accordance with the foregoing objective of the present invention, theinvention provides a method for fabricating gate oxide that includesproviding a silicon substrate, performing an oxidation process, andperforming an annealing process. The oxidation process includes a dilutewet oxidation in an environment filled with oxygen, moisture, andnitrogen at a temperature of about 750°-900° C. to form a gate oxidelayer over the substrate, wherein the volume of filled nitrogen is about6 to 12 20 times of the volume of filled moisture. The gas used in theannealing process includes nitrogen at a temperature of about 800°-1200°C.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the followingdetailed description of the preferred embodiments, with reference madeto the accompanying drawings, wherein:

FIG 1 is a flowchart showing a conventional method for fabricating gateoxide; and

FIG 2 is a flowchart showing the method for fabricating gate oxide of apreferred embodiment according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides a new method for fabricating gate oxide. FIG 2 isa flowchart showing the method for fabricating gate oxide according tothe invention.

FIG. 2 shows step 110, in which a dilute wet oxidation process isperformed in a environment filled with oxygen, moisture, and dilute gasat a temperature of about 750°-900° C. to form a gate oxide layer. Thedilute gas includes inert gas such as nitrogen. The volume of the dilutegas is about 6-126-20 times of the volume of the moisture. Then, anannealing process is performed on the gate oxide layer to eliminate thestress within the gate oxide layer caused by defects within the gateoxide. The annealing process is performed in the presence of a gas, suchas N₂O, NO, or other nitrogen base gases, at a temperature of about800°-1200° C.

The gate oxide with a thickness of about 40 Å formed by the method ofthe foregoing preferred embodiment according to the invention has ameasured Q_(bd) equal to or less than 15 Coulombs per square centimeter.This is about three times that of a conventional gate oxide. Such a gateoxide also has a leakage current of about 16×10⁻⁸ amperes under a gatevoltage of 3 2 volts, which is about half the leakage current of aconventional gate oxide.

It is obvious that the method for fabricating gate oxide according tothe invention improves the electrical quality of gate oxide by raisingthe Q_(bd) and reducing the leakage current.

The invention has been described using exemplary preferred embodiments.However, it is to be understood that the scope of the invention is notlimited to the disclosed embodiments. On the contrary, it is intended tocover various modifications and similar arrangements. The scope of theclaims, therefore, should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements

1. A method for fabricating gate oxide, the method comprising: providinga silicon substrate; forming the gate oxide by performing a dilute wetoxidation process with additional nitrogen and moisture, wherein thenitrogen has a volume of about 6 to 12 20 times the moisture's volume;and performing an annealing process with a nitrogen base gas on the gateoxide.
 2. The method of claim 1, wherein the dilute wet oxidation isperformed at a first temperature of about 750°-900° C.
 3. The method ofclaim 1, wherein the annealing process is performed at a secondtemperature of about 800°-1200° C.
 4. The method of claim 1, wherein thenitrogen base gas includes N₂O.
 5. The method of claim 1, wherein thenitrogen base gas includes NO.